Determining the location of the source of an electromagnetic signal is important for signal intelligence gathering, targeting, and weapon delivery directing, navigation, etc. The expanding and increasingly innovative use of the radio frequency spectrum across a wide range of environments requires an improvement in the existing capabilities in direction sensing in order to detect signals at longer distances and lower powers and to provide higher precision in directionality determination, all while evading detection by potentially hostile forces.
Conventional radio frequency direction finding (RFDF) systems often employ an array of multiple electrically resonant antennas. The direction of an incoming signal is determined based on a phase difference of an incoming signal as received at each of the antennas.
This may be understood with reference to prior art FIG. 1 which shows an example of an antenna array used for radio frequency direction finding. As shown in FIG. 1, the array 100 includes multiple antennas 10A, 10B, 100 and 10D that receive an incoming signal of wavelength λ at an incident angle θ relative to the array 100. The antennas are spaced apart, such that there is a distance d1 between antennas 10A and 10B, a distance d2 between antennas 10A and 100, and a distance d3 between antennas 10A and 10D, and so on and so forth up to a distance dn-1 for n antennas. As shown in FIG. 1, when an incoming electromagnetic signal arrives at the array, the phase (or timing) of the signal as received at each antenna 10A, 10B, 100 and 10D is detected by phase detectors 20A, 20B, 20C, and 20D, respectively.
The differences between the detected phases of the signal as received at antenna 10A and each of the antennas 10B, 100 and 10D are determined by phase comparators 30A, 30B and 30C through 30n-1 and output as phase difference outputs ø1, ø2 and ø3 through øn-1, respectively.
In general, the relationship between the phase difference, Δø, of an electromagnetic signal received by two antennas separated by a distance d is given as a function of wavelength, λ, the distance, and angle of arrival (θ), where:
                    θ        =                              sin                          -              1                                ⁡                      [                          λ              ⁢                                                          ⁢                                                                    Δ                    ⁢                                                                                                  ∅                                /                2                            ⁢              π              ⁢                                                          ⁢              d                        ]                                              (        1        )            such that the phase difference is given by:
                    Δ∅        =                              (                          2              ⁢              π              ⁢                                                          ⁢                              d                /                λ                                      )                    ⁢          sin          ⁢                                          ⁢          θ                                    (        2        )            
Applying these equations to the antenna array depicted in FIG. 1, the angle of arrival θ, which corresponds to the direction of the incoming electromagnetic signal, can be derived from the phase difference outputs ø1, ø2 and ø3 of the respective phase comparators 30A, 30B, and 30C. This calculation can be typically performed by a digital signal processor.
Existing radio frequency direction finding (RFDF) systems built from conventional electrically resonant antennas such as those depicted in FIG. 1 are necessarily frequency range limited. This means that to cover a full radio frequency (RF) spectrum, an RFDF system must include many antenna and receiver chain elements. Also, the physical dimensions of such systems are subject to ¼ wavelength baseline restrictions. This means that high frequency (HF) direction finding and similar long wavelength sensing systems are necessarily physically large and subject to detection and disruption by potentially hostile forces.
There is thus a need for a phase difference discrimination system that can be used for broadband direction finding and can be implemented on a small platform.